The invention relates to a bonded assembly, such as a metal/plastic wiper blade assembly, and to a method of preparing same.
Wiper blades are used in a printer and copier products to clean surfaces, such as electrically charged drums. A typical design for such a structure utilizes a blade of a flexible plastic material, such as a reinforced polyurethane, which is mounted on a support, conventionally by bonding using a hot melt adhesive. Such hot melt adhesives have a fast setting time, which allows the assemblies to be produced rapidly and efficiently. However wiper blade assemblies are subjected to peel stresses in use and hot melt adhesives have not given entirely satisfactory peel performance in this application. Accordingly it would be desirable to have an adhesive system which gave improved peel strengths in use while preserving the benefits of an assembly technique adapted for high speed production. Desirably the system would provide adhesive set times of less than one minute, preferably about 30 seconds or less.
As alternatives to hot melt adhesives, a large variety of curable adhesives are known. Many such adhesives will yield good tensile strengths, but are ordinary at best resisting peel stresses. This peel stress resistance is a particular problem with fast curing systems, such as cyanoacrylate adhesives or acrylic systems activated with a primer. Primer activation is also undesirable for its added complexity.
Anaerobic and/or epoxy adhesives may provide high peel strengths, but typically require relatively long setting times.
Within the curable adhesive art it is known that adhesives based on acrylate monomers generally cure faster when exposed to UV irradiation and produce less rigid cured polymers than adhesives employing methacrylate monomers. Accordingly, although UV curing methacrylate-based adhesives are known, it is generally much preferred to use acrylates when fast cure and flexible cured properties are desired.
U.S. Pat. No. 4,309,526 (Baccei et al) describes unsaturated curable poly(alkylene)ether polyol based urethane resins which give flexible cured properties with both methacrylate and acrylate unsaturation. Compositions which cure by UV irradiation are described as well as compositions which cure anaerobically.
While the state-of-the-technology provides various adhesive solutions to a variety of application requirements, it is not believed that metal/plastic wiper blade assemblies, and the issues presented during manufacture thereof, have been adequately addressed prior to the invention described herein.
The inventors have discovered that wiper blade assemblies can be bonded with a urethane (meth)acrylate based adhesive which has been modified to provide both UV and anaerobic curing mechanisms. The adhesive provides both desired rapid setting time when irradiated through a transparent or translucent substrate, and desirable high peel strengths.
Accordingly, in one aspect the invention comprises a method of preparing a bonded assembly of two substrates at least one of which is transparent or translucent to UV or visible light, the method comprising
a) applying, to at least one of the substrates, a photo/anaerobic dual cure composition comprising:
i) a (meth)acrylate-capped urethane oligomer,
ii) at least one (meth)acrylate diluent monomer,
iii) an anaerobic curing system, and
iv) a free-radical photoinitiator;
b) joining the two substrates
c) irradiating the adhesive through said transparent or translucent substrate with light of a wavelength effective to activate the photoinitiator for a time sufficient to at least fixture the adhesive, and then
d) allowing the assembly to further cure at ambient conditions without for at least 12 hrs without subjecting the fixtured assembly to substantial peel stresses.
Further aspects of the invention include the adhesive used in the inventive method and bonded assemblies prepared by the inventive method, especially such assemblies in which the transparent or translucent substrate is a flexible polyurethane. The inventive method can be used to prepare wiper blade assemblies with an assembly time as fast as a hot melt adhesive and yet to provide peel strengths in use which are substantially higher.